Elsevier

Ceramics International

Volume 38, Issue 7, September 2012, Pages 5527-5533
Ceramics International

Influence of additives on microstructure and property of microarc oxidized Mg–Si–O coatings

https://doi.org/10.1016/j.ceramint.2012.03.068Get rights and content

Abstract

Ceramic coatings were fabricated on ZK60 magnesium alloy substrate by microarc oxidation (MAO) in Na2SiO3–KOH base electrolyte with four kinds of additives (i.e. KF, NH4HF2, C3H8O3 and H2O2). The effects of these additives on microstructure and property of coatings were investigated. The surface morphology, phase composition and corrosion resistance of the ceramic coatings were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion test respectively. It is found that different additives can change the spark discharge phenomenon during microarc oxidation. It is proved that both potassium fluoride (KF) and ammonium bifluoride (NH4HF2) promote discharge and accelerate reaction while the introduce of glycerol (C3H8O3) leads to the refining of sparks and reduction of thermal effects. Results also demonstrate that the introduce of hydrogen peroxide (H2O2) contributes to the increase of coating surface roughness and enlargement of surface micropore size. XRD results indicate that the ceramic coatings are mainly composed of Mg2SiO4, MgSiO3 and SiO2. The introduce of H2O2 hinders the reaction between SiO2 and MgO and creates favorable conditions for the formation of the MgO phase. The ceramic coatings formed in base electrolyte containing 7 g/L NH4HF2 and 5 mL/L C3H8O3 exhibit the highest corrosion resistance.

Introduction

By virtue of a unique combination of low density, high specific strength and good electromagnetic shielding characteristics [1], magnesium alloys show promise for use in many industrial applications. However, they are restricted in practice by their poor corrosion resistance [2], [3], [4], [5]. There are many coating technologies available for corrosion protection and surface hardening of Mg alloys, such as anodizing [6], [7], chemical conversion [8], electroplating, thermal spraying [9], biomimetic approach [10] and electrochemical deposition [11], [12]. However, with inevitable limitations, they are not adequate for use in harsh service conditions, so proper surface treatment which can produce a relatively thick, dense, hard and anti-corrosive coating is required to improve properties of Mg alloys.

Microarc oxidation, based on conversional anodic oxidation technology, is a new promising surface treatment method to form in situ-grown ceramic coatings directly on so-called value metals, such as magnesium, aluminum, titanium and their alloys in weak-alkaline electrolyte without chromates at the voltage of alternating current up to 1000 V, usually accompanied by sparking and intensive gas evolution phenomenon at the anode surface [13], [14], [15], [16], [17]. The method of MAO has a lot of advantages over other surface treatments, e.g. excellent adhesion between coating and substrate, low energy consumption, easy controlling for processing and ecology-friendly process and products [18], [19], [20], [21].

The structure and property of microarc coatings depend on many factors, such as substrate materials, electrolyte compositions, electrical parameters and so on. It is found that the electrolyte compositions play a key role in the MAO process [22], and some additives, such as KF, NH4HF2, C3H8O3 and H2O2, are favorable to the MAO process and promising for the formation of ceramic coatings with good properties. The influence of C3H8O3 on the characteristics of MAO coatings on AZ91D magnesium alloy has been studied [23]. However, research with MAO process on ZK60 magnesium alloy with introduce of KF, NH4HF2, C3H8O3 and H2O2 in the base electrolyte is seldom conducted.

In this paper, KF, NH4HF2, C3H8O3 and H2O2 were added into the silicate electrolyte one by one. Ceramic MAO coating with different performance were formed on ZK60 magnesium alloy. The effects of these additives on discharge phenomenon, coating appearance, phase composition and corrosion resistance were evaluated and a modified additive formula was obtained.

Section snippets

Samples and solutions

Rectangular specimens (with dimensions of 8 mm × 10 mm × 12 mm) made of wrought magnesium alloy ZK60 (Zn 6%, Zr 0.45%, Mg balance) were used as the substrate material in the present study. Prior to the oxidation treatment, the samples were mechanically polished with carborundum waterproof abrasive paper up to 1000 grit, degreased with acetone following by rinsing with distilled water, and then immersed in electrolyte for MAO treatment after dried in warm air.

The base electrolyte was prepared from

Phenomenon of MAO process

It was observed that the discharge spark color was different with introduce of different additives in base electrolyte during MAO process. The variation of the spark color represents reaction energy change, and the energy of white spark is much higher than that of orange one. The higher the energy is released, the denser the coating is formed. When KF alone was added into the base electrolyte, the discharge sparks were white and well distributed all over the sample surface. When NH4HF2 alone

Conclusion

  • (1)

    The four kinds of additives (i.e. KF, NH4HF2, C3H8O3 and H2O2) play different roles in the formation of MAO coatings. Both KF and NH4HF2 promote discharge and accelerate reaction. C3H8O3 can effectively reduce the poor thermal effect produced by oxidation reaction of magnesium alloys as stabilizer. The introduce of H2O2 results in oxygen libration and makes coating surface rough.

  • (2)

    The MAO coatings are mainly composed of Mg2SiO4, MgSiO3 and SiO2 in spite of different additives, but the intensity

Acknowledgments

This work is part of a research program financed by Science and Technology Supporting System Item of Resource-conserving Society of Shandong Province (Project No. 2007JY05) and the Development Project of Science and Technology of Shandong Province (Project No. 2010GSF10627).

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